1. Field of the Disclosure
The present disclosure relates to a process for preparing titanium dioxide, and in particular to the in-situ formation of chlorides of silicon such as silicon tetrachloride and chlorides of aluminum such as aluminum trichloride in the preparation of titanium dioxide.
2. Background of the Disclosure
Titanium dioxide pigments are prepared using either the chloride process or the sulfate process. In the preparation of titanium dioxide pigments by the vapor phase chloride process, titanium tetrachloride, TiCl4, is reacted with an oxygen containing gas at temperatures ranging from about 900° C. to about 1600° C., the resulting hot gaseous suspension of TiO2 particles and free chlorine is discharged from the reactor and must be quickly cooled below about 600° C., for example, by passing it through a conduit, i.e., flue, where growth of the titanium dioxide pigment particles and agglomeration of said particles takes place.
It is known to add various substances, such as silicon compounds and aluminum compounds, to the reactants in order to improve the pigmentary properties of the final product. Aluminum trichloride added during the process has been found to increase rutile in the final product, and silicon tetrachloride that becomes silica in the final product has been found to improve carbon black undertone, CBU, particle size and pigment abrasion.
U.S. Pat. No. 3,434,799 discloses the use of silicon metal in the production of TiO2; however, experience of adding silicon metal in the AlCl3 generator shows poor kinetics for the conversion of silicon to SiCl4. For direct addition of silicon metal to the TiO2 reactor, it would require a very finely divided silicon metal which would be expensive to produce and create a fire or explosion hazard.
U.S. Pat. No. 3,642,442 discloses a method for adding aluminum and silicon to the process by generating a gaseous plasma and discharging particles to the process oxygen stream. A shortcoming of this patent is that the electrodes have a limited life and must be replaced on a routine basis requiring the TiO2 process to be shut down.
The transport, storage and handling of silicon tetrachloride creates issues because the material is highly toxic and silicon tetrachloride handling equipment adds costs to the process.
A need exists for a low cost approach for adding silicon tetrachloride to the process without the need for transportation of a highly toxic material, special processing equipment and expensive silicon tetrachloride facilities. A need also exists for a process wherein the feed to the AlCl3 generator can be continuous with no shutdown required for maintenance on the electrodes or equipment.